8-22] THE ANGLE MEASUREMENT STABILIZATION PROBLEM 429 



the controlled gain variation to the order of 20 db per stage, will minimize 

 these gain variations. When a gain control range in excess of 50 or 60 db 

 must be provided, it is frequently necessary to control the first stages of the 

 radar receiver. When this is done, control voltage must be provided in a 

 manner that causes the least deterioration of signal-to-noise ratio. 



In addition to grid-1 control of the amplifier stages, grid-3 or plate and 

 screen control is sometimes employed. Grid-3 control provides minimum 

 third-order distortion, but the screen dissipation is generally excessive when 

 the tubes are operated with reasonable gain bandwidth factor. As a result 

 the best compromise is grid-1 control of remote or semiremote cutoft tubes. 



8-22 THE ANGLE MEASUREMENT STABILIZATION 

 PROBLEM 



In airborne radars the measurement of target angular position is compli- 

 cated by the angular motion of the airborne platform. This paragraph will 

 discuss the general features of this problem and the approaches that are 

 employed to solve it. Subsequent paragraphs will show how a specific 

 stabilization prpblem — the AI radar search and track stabilization problem 

 — might be approached. The techniques and lines-of-reasoning used in 

 this example are typical of those which must be employed for the solution 

 of any airborne radar stabilization problem. 



The essential features of the problem are illustrated by the simple 

 one-dimensonal representation of Fig. 8-23. The space pointing direction 

 of the antenna Atl is made up of two components: (1) the angle At of the 

 antenna with respect to the aircraft and (2) the space orientation angle of 

 the aircraft A a. Thus changes in the orientation of the aircraft — due 

 either to maneuvering or disturbances from wind gusts, etc. — will cause 

 corresponding changes in the space pointing direction of the antenna. 



From a tactical standpoint, this situation is undesirable. The line of 

 sight from the radar to the target is relatively independent of radar aircraft 

 orientation (neglecting long-term kinematic effects, it is completely 

 independent). Thus, the effect of aircraft platform motion is to degrade 

 the radar's ability to measure the target's position in space. 



The term angle stabilization refers to the family of techniques employed 

 to isolate the radar measurements from the degrading influences of aircraft 

 motions. These techniques fall into two general classes: (1) data stabiliza- 

 tion and (2) antenna stabilization. 



Data Stabilization. With this technique, no changes are made to 

 the basic control loops illustrated in Fig. 8-23. The effects of aircraft 

 motion are compensated in the data-processing system by correcting the 

 antenna angle measurements by appropriate functions of the measured 

 platform motion. 



